Hysteresis or imprinting of mirror deflection amplitude in Spatial Light Modulators (SLM), which leads to a shift in a required address voltage for a preferred mirror deflection, is a problem since it reduces the accuracy of the mirror deflection. This shift depends on an amount of time that the mirror has been deflected, an amplitude of the deflection and a time that the mirror has had to relax after said deflection. The time constants for these phenomena are in the region of minutes/hours making it virtually impossible to predict and compensate the required address voltages for an arbitrary pattern in a mask writer or a direct writer using a state of the art SLM technology.
A problem with a compensation for the imprinting is that the mirrors in the SLM are loaded with data row by row, always from the same direction. This means that with all mirrors in a fully deflected state the first row pixels will have a very high duty-cycle while in the last row they will have a very low duty-cycle and the intermediate rows will have intermediate duty-cycles. It is therefore also impossible, or at least very difficult, to use a collectively addressable pixel counter electrode as a balance for the imprinting effect unless the SLM is redesigned to load the data from the first and the last row every second time to average the duty-cycles. Even then the imprinting would still be pattern dependent and it would be necessary to often calibrate the mirrors or leave the system idle for a period of time to let the mirrors in the SLM relax.
Therefore, there is a need in the art for a method and an apparatus with reduced imprinting effect.